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Recapping Things Again
Let's go over things again to make sure we have this idea of DCC clear in our minds.
A DCC System is a Bunch of Computers Connected Together
Firstly, you have to make a paradigm shift in your thinking. DCC is not a power pack that you are used to seeing when you operate your train in analog.  Think of DCC as a bunch of computers that are connected together in a local area network.  Some computers have more power and capabilities than other computers.  I'll use the Digitrax Zephyr as an example.

The Command Station
In the case of DCC, the computer with the most power and capabilities is called the command station.  The command station is like a central server that receives messages and reroutes those messages to other computers.  

The Throttle
Another type of computer with lots of capabilities is called the throttle.  In the Digitrax Zephyr, one of the throttles happens to "sit" on top of the command station.  For the Zephyr (or any of the Digitrax command stations), you could just as easily run some

"telephone lines" around your layout and plug another type of throttle (like the Digitrax DT400 or DT300 throttle) into these phone jacks and run your locos. 
The Decoder
The third type of computer is called the decoder.  This is a PC-type board that is wired into your favourite loco so that it can receive commands/messages from the command station.  (Some decoders look like a PC board, while others look as if they've been shrunk-wrapped in coloured plastic.)  Different types of decoders have different capabilities.  All decoders will run the loco forwards and backwards.  If I send a message from my throttle via the command station to the decoder in your locomotive, your locomotive will respond (move forwards, backwards, stop, speed up slow down, etc).  

Most decoders will turn the headlights on and off (if headlights are installed in the loco).  I like to program my decoders so that the front light automatically comes on when I'm moving forward and the rear light comes on when I'm moving backwards.  This helps me to know which way the loco is going to move when I start up the loco.  But I can also send a message from my throttle via the command station to the decoder to turn the lights off or to dim them.  

Some decoders, for example, a Soundtraxx decoder, will automatically simulate the chuff of a steam loco, and the bell and whistle can be controlled from the throttle.  
The Booster
Now, to run a loco, we need to have a larger amount of power on the tracks than what we have in our telephone wires.  And we need to send some signals down the tracks so that the decoders in the locos can respond to the commands that we send to the decoder (like move forward, reverse, speed up, slow down, etc).  So, we need another type of computer which is called a booster. The booster takes the signals received from the command station and superimposes these signals onto a higher voltage and amperage and sends the signals down the tracks.  If the signal is addressed to the decoder in your locomotive, then your loco will respond.
The Command Station/Booster
It just so happens that the command station and the booster are combined in one box in the Digitrax Zephyr.  Likewise with the Digitrax DB150 Empire Builder and the DCS 100 Chief we have one box.

Some "Laws of DCC"

I started off trying to understand DCC about 2 years ago.  There's a lot of information out there, but there's not a lot of information that will explain what you should be looking for in a DCC system.  How much of the product is interchangeable?  What are the consequences if you commit to one brand?  What do I get if I buy a DCC set?  What else am I going to need?  How do I hook this thing together.  What we'll try and do on the next few pages is put some thoughts down on page.
(The usual disclaimers)  These thoughts are mine and mine alone.  The only reason I use Digitrax in the examples is because that's what I use for my DCC system.  Just because I quote Digitrax as an example is not an endorsement for Digitrax.  I also use Atlas and Soundtraxx decoders and a lot of other components.  If, for any reason, you take these thoughts and suggestions and make them part of your thoughts, you assume complete liability.  (End of Disclaimers)

Law #1 - The Law of Decoders - Different brands of decoders can be used on different types of DCC systems. 
Each locomotive you are running on the track should have a "decoder" installed in it. A decoder is a small computer on a printed circuit board (with some other electrical components) that is installed into the locomotive so that it can become a part of our networked system.  There are many brands and sizes of decoders, all of which can be used on any "DCC system" (eg Atlas, NCE, Lenz, Soundtraxx decoders can all be used on a Digitrax "DCC system". Or, Digitrax, Atlas, Soundtraxx decoders can all be used on a Lenz "DCC system"). Decoders start at around $30. 

Law #2 - The Law of Interchangeability - Very few of the components (command stations, boosters, throttles) of a "DCC system" from Manufacturer A can be used with the components from the "DCC system" of Manufacturer B.  (Exception - Law#1 - The Law of Decoders over-rides the Law of Interchangeability) 
To control the locos on your layout, you will require a "DCC system". This typically consists of a power supply, a command station, a booster, and a number of throttles. Not all of the components in one system can be interchanged with components from another system. For example, a Digitrax DT300 throttle can't be used on an Atlas "DCC system". Some components on an Atlas DCC system can be used on a Lenz DCC system. None of the components on a Digitrax system can be used on the Atlas system. The degree of interchangeability will depend on the manufacturer. The only exception to this law are decoders - these are covered by DCC System Law #1. 

Law #3 - Maximum Locos - The maximum number of locos on a "DCC system" is determined by the total current draw (amps) and the maximum number of locos that can be controlled by your "DCC system".  It will be the lesser of these two factors.   (I'll only talk about locos.  So wherever you see "locos", that should read "locos and electrical devices connected to your DCC system".)
The number of locos you can run on a layout at one time will depend on two things.  The first is the number of locos that the "DCC system" can control. For example, the Digitrax Empire Builder can control up to 22 locos. The Digitrax Chief can control up to 122 locos. The Atlas Commander can only handle 7 throttles. This limitation is sometimes referred to as "slots" (eg "The Digitrax Empire Builder has 22 slots"). (Without confusing the issue, if you connect or "consist" a number of locos together into one train, your "DCC system" sees them as one loco.)
The second thing that determines the number of locos you can run on a "DCC system" is the "size" of the command station/booster and the total current draw of the locos (and other electrical devices) on your layout. If the maximum output amperage of your "DCC system" is 5 amps and each loco requires 0.5 amps to run, you can only run 10 locos on your layout (0.5 amps x 10 locos = 5.0 amps which is the maximum output of your "DCC system"). To run more locos on your layout, you would have to add extra boosters - a subject we won't get into at this stage. 
Now, what if my locos draw a lot less amperage. For example, if each loco only draws 0.1 amps, you might be able to run a maximum of 50 locos (0.1 amps x 50 locos = 5.0 amps). However, if your "DCC system" has a maximum of 22 slots, then you can only run 22 locos on your layout. 
To recap - Maximum locos at one time is the lesser of 
  • the total current draw (amps) or 
  • the maximum number of "slots" on your "DCC system". 

Law #4 - Pocketbook Reality - The DCC system you buy will depend on; a) how much money you can afford to spend, and b) what features would you like to have more of.  The more features you want, the more it's going to cost. 
While we're talking about maximums, the reality will be "How many trains are you likely to run at one time on your layout?"  "What is the maximum number of throttles I'll ever use on my layout?"  While you might buy a "DCC system" that can run a maximum of 999 locos, will you ever run that many locos at one time on your layout?  The difference in cost between a 999 loco system and a 9 loco system could be several hundred dollars.   You'll see lots of debate about the merits of System A over System B.  However, a lot of this is like comparing a top-of-the-line automobile to a small sub-compact.  Like buying a car, the reality will be how much money can you afford to spend. 

Law #5 - The Empty Purse  Buy as many features on a DCC system as you can afford.  This is sort of a corollary to the Law of the Pocketbook. 
There's always hesitation to go for the next higher level of DCC system offered by a manufacturer.  Don't hesitate!  Go for it if you can afford it.  Talk to anyone who bought an entry-level system.  They'll tell you they should have bought the more sophisticated system. 

Law #6 - The Law of the Upgrade  - When buying a DCC system, make sure you understand whether your DCC system can be upgraded with other components offered by the same manufacturer or with other components offered by other manufacturers. 
Another factor to consider when buying a DCC system is whether you can expand your system later on without having to scrap your initial investment.  For example, you might start off with the Empire Builder (DB150 command station/booster and DT300 throttle) which can handle up to 22 trains (22 slots). 
Later on, as you invite your friends in for operating sessions, or as you run more locos, you might want to expand to the DCS100 command station/booster.  Will I be able to use my DT300 throttle with the DCS100?  What about the DB150?  Digitrax has made all of their DCC systems so that you can upgrade from one level to the other (in this case, DB150 with 22 slots to DCS100 with 122 slots) and not lose all of your investment.  The DT300 throttle can be used on both the DB150 and the DCS100.  The DB150 can be used as a booster for a power district.  What will it take to upgrade your DCC system?

Law #7 - Patience is a Virtue - Take your time to try and understand what DCC is all about.  Get as much information as you can.  Draw up some charts that compare features.  Decide what features you want in your DCC system. 
There's not a lot of information that explains what to look for when comparing one DCC system with another, or even comparing a manufacturerer's entry level system with their top-of-the-line system.  It gets very confusing trying to understand and absorb all of this information.  There are some websites which do compare systems.  However, you're the one who has to decide what's best for you. 
As you get more information, you might become frustrated with all the info being thrown at you. Don't give up. Keep absorbing and trying to understand what DCC is all about, what are the differences and similarities between the various DCC systems, what are the features you would like, what DCC systems can provide you with those features, and - the ultimate question -  what can I afford. 
Talk to friends and the people in the chat groups. Try visiting a club that uses DCC. Talk to a hobby shop that is familiar with DCC, download the manuals from the manufacturers' websites.  On my modular railroad club we set up layouts over 300' long. At our last setup we used 6 Digitrax DCS100 Chiefs and DB150 Empire Builders. We use every brand of decoder that is manufactured. We had over 24 throttles in use at one time. I lost count of the number of locos on the layout. 
Two years ago, I didn't know what DCC was. Today I know a lot more about the subject and I'm still learning - thanx to chat groups, DCC websites, friends who have DCC, hobbyshops that are knowledgeable, and MR clubs that use DCC. 
As you absorb more information, this subject of DCC will start to get clearer. Once you've used DCC, you'll wonder what you ever did without it.
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